For some time, image intensifier tubes have been used in a variety of applications for direct viewing at low light levels and near infrared regions of the spectrum. Image intensifier tubes have been used in a variety of military, scientific and industrial applications where assistance in viewing objects at low light levels is necessary. For example, the devices are used in military applications to view dimly illuminated targets.
Image intensifier tubes are electro-optical devices which convert a low energy visible or invisible radiant image into an electron image by means of a photocathode. This image is increased in energy and reconstructed by a focusing electric field on a phosphor screen. The radiant image is reconverted on the phosphor screen to a brighter image of varied or like size.
The development of image intensifier tubes has progressed through several generations of units.
In first generation image intensifier tubes, the low light level image is incident upon a fiberoptic face plate which focuses the image on a photocathode where the photon image is converted into an electronic one. The electrons are accelerated toward a phosphor screen while the spatial information is maintained by the electron optics. The accelerated electrons strike the phosphor, thus indicating an amplified image. Generally, three stages of intensifier stages are utilized in the first generation type.
After many years of development, a second generation image intensifier tube was developed. This second generation unit incorporated a micro-channel plate comprised of a bundle of discrete hollow glass tubes or channels capable of amplifying an electron image by many orders of magnitude. As in the first generation of image intensifier tubes, the electron image in the second generation units are generated by a photocathode in response to the incident radiation image. However, the multiplied electron image from the micro-channel plate is directed onto a phosphorus screen for providing an intensified display of the sensed radiation image without the need for stages of amplification.
Research and development efforts for the past several years have been directed in developing new materials for light sensing and detection devices and have produced a third generation image intensifier tube which uses a glass cathode substrate. The most promising materials for such third generation image intensifier tubes are the compounds of gallium arsenide, aluminum gallium arsenide and indium gallium arsenide. Each of these materials is an electro-luminescent type material. These materials can be grown into crystal wafers and bonded to a glass cathode substrate more readily than a fiberoptic material, resulting in a better crystal yield. A successful technique of fabrication utilizes the above-mentioned compounds to grow epitaxial layers on single crystal substrates by liquid phase techniques. The liquid phase method is well known and highly developed for small area growths, for example, on the order of 18 milimeter tubes. Thus, the third generation intensifier tube has developed into a wafer intensifier tube that incorporates a microchannel plate which has an ion barrier film of aluminum oxide and a gallium arsenide photocathode. However, internal reflection of off-axis light in the glass cathode substrate reduces the effectiveness of such a design. Therefore, a need exists for a device for preventing off-axis or stray light from being internally reflected in the glass cathode substrate.